run acceptance tests with proxy in between

[individual-it]

acceptance tests should also run through a proxy server, specially interesting for federation tests when one of the servers is behind a proxy.
e.g. we could run some federation smoke tests with one or both servers behind a proxy

see also #34101 specially #34101 (comment)

[phil-davis]

drone starlark has some pipelines for webUI and API smoketests with proxy.
https://github.com/owncloud/core/blob/master/.drone.starlark#L181

What else to do here? Federation+proxy?

[individual-it]

proxy with credentials and proxy with SSL would be interesting